SYNTHESES AND ELECTROCHEMISTRY OF SULFUR-BRIDGED INCOMPLETE CUBANE-TYPE MIXED-METAL CLUSTERS OF MOLYBDENUM(IV) AND TUNGSTEN(IV) - X-RAY STRUCTURES OF [MOW2S4(H2O)(9)](CH3C6H4SO3)(4)CENTER-DOT-9H(2)O, [MO2WS4(H2O)(9)](CH3C6H4SO3)(4)CENTER-DOT-9H(2)O, NA-2[MOW2S4(HNTA)(3)]CENTER-DOT-5H(2)O, AND NA-2[MO2WS4(HNTA)(3)]CENTER-DOT-5H(2)O
T. Shibahara et al., SYNTHESES AND ELECTROCHEMISTRY OF SULFUR-BRIDGED INCOMPLETE CUBANE-TYPE MIXED-METAL CLUSTERS OF MOLYBDENUM(IV) AND TUNGSTEN(IV) - X-RAY STRUCTURES OF [MOW2S4(H2O)(9)](CH3C6H4SO3)(4)CENTER-DOT-9H(2)O, [MO2WS4(H2O)(9)](CH3C6H4SO3)(4)CENTER-DOT-9H(2)O, NA-2[MOW2S4(HNTA)(3)]CENTER-DOT-5H(2)O, AND NA-2[MO2WS4(HNTA)(3)]CENTER-DOT-5H(2)O, Inorganic chemistry, 33(2), 1994, pp. 292-301
Reduction of a equimolar amount of (NH4)(2)WS4 and Na-2[Mo2O2S2(cys)(2
)].4H(2)O with NaBH4 gives a mixture of sulfur-bridged clusters, chrom
atographic separation of which gives novel incomplete cubane-type moly
bdenum-tungsten mixed-metal clusters [MoW2S4(H2O)(9)](4+) and [Mo2WS4(
H2O)(9)](4+). From the aqua clusters solid samples [MoW2S4(H2O)9](CH3C
6H4SO3)(4).9H2O (MoW(2)aq) and [Mo2WS4(H2O)9] (CH3C6H4SO3)(4).9H2O (Mo
(2)Waq) were isolated, and the derivatives Na-2[MoW2S4(Hnta)(3)].5H(2)
O (MoW(2)nta), and Na-2[Mo2WS4(Hnta)(3)].5H(2)O (Mo-2-Wnta) were obtai
ned (H(3)nta = nitrilotriacetic acid). Crystal structures of the four
clusters were determined. Compound MoW(2)aq crystallized in the tricli
nic space group P ($) over bar 1 with a 15.303(3) Angstrom, b = 16.682
(6) Angstrom, c = 12.055(3) Angstrom, alpha = 96.02(3)degrees, beta =
108.58(2)degrees, gamma = 102.16(2)degrees, V = 2802.1(15) Angstrom(3)
, Z = 2, R = 4.77%. Compound Mo(2)Waq crystallized in the triclinic sp
ace group P ($) over bar 1 with a = 15.327(5) Angstrom, b = 16.704(5)
Angstrom, c = 12.057(5) Angstrom, alpha = 95.64(3)degrees beta = 108.8
2(3)degrees, gamma = 102.41(2)degrees, V = 2806.2(18) Angstrom(3), Z =
2, R = 5.98%. Compound MoW(2)nta crystallized m the monoclinic space
group P2(1)/a with a = 21.754(5) Angstrom, b = 12.733(5) Angstrom, c =
13.374(5) Angstrom, beta = 101.95(3)degrees, V = 3624.4(21) Angstrom(
3), Z = 4, R = 3.90%. Compound Mo(2)Wnta crystallized in the monoclini
c space group P2(1)/a with a = 21.741(5) Angstrom, b = 12.714(5) Angst
rom, c = 13.384(6) Angstrom, beta = 102.10(3), V = 3617.3(24) Angstrom
(3), Z = 4, R = 4.74%. Molybdenum and tungsten atoms in the four cryst
als are statistically disordered. Peak positions of the two big bands
in each infrared spectrum of the four aqua clusters, [Mo3S4(H2O)9](CH3
C6H4SO3)(4).9H(2)O (Mo(3)aq), Mo(2)Waq, MoW(2)aq, and [W3S4(H2O)(9)](C
N3C6H4SO3)(4).9H(2)O (W(3)aq), in the 550-400-cm(-1) region shift to l
ower wavenumbers when the molybdenum atom is replaced by tungsten. The
bands are tentatively assigned to nu (metal-OH2) (at higher wavenumbe
r) and nu (metal(3)-S) (at lower wavenumber). Binding energies of moly
bdenum (3d(3/2) and 3d(5/2)) and tungsten (4f(5/2) and 4f(7/1)) are ob
tained from XPS spectra of the clusters with Mo3-nWnS4 cores (n = 0-3)
. The binding energies of Mo in Mo(3)aq and Mo(3)nta change little on
the replacement of Mo with W, and those of W in W(3)aq and W(3)nta cha
nge little on the replacement of W with Mo, also: these phenomena can
be explained by the softness of the bridging sulfurs, which act as a b
uffer for the electron density changes on the Mo and W atoms. Current-
sampled de polarograms and cyclic voltammograms of MoW(2)nta and Mo(2)
Wnta show three consecutive one-electron reductive steps in alkaline s
olution (pH 11.4): E(1/2) (V vs Ag/AgCl) = -0.84, -1.40, -1.78; -0.73,
-1.22, -1.66, respectively. These steps correspond to the change of o
xidation states of the three metals in each cluster: (IV, IV, IV) -->
(IV, IV, III) --> (IV, III, III) --> (III, III, III). Electronic spect
ra of one-electron reduction products, the oxidation state being (IV,
IV, III), obtained by bulk electrolysis of MoW(2)pta and Mo(2)Wnta hav
e been reported for the first time. The half-wave potentials E(1/2) ar
e significantly dependent on the cluster metals. In all the reduction
processes the [MonW3-nS4(Hnta)(3)](2-) clusters (n = 0-3) are easily r
educed with the increase in the numbers of Mo(n) in the cluster. The p
roceding reduction center of the molybdenum-tungsten mixed-metal clust
ers is mainly on the Mo atom(s).